CS5531-AS datasheet(40/52 Pages) CIRRUS

40 / 52 page

CS5531/32/33/34

DS289PP5

2.9. Digital Filter

The CS5531/32/33/34 have linear phase digital fil-

ters which are programmed to achieve a range of

output word rates (OWRs) as stated in the Channel-

Setup Register Descriptions section. The ADCs use

aSinc5 digital filter to output word rates at 3200

Sps and 3840 Sps (MCLK = 4.9152 MHz). Other

output word rates are achieved by using the Sinc5

filter followed by a Sinc3 filter with a programma-

ble decimation rate.Figure 16 shows the magnitude

response of the 60 Sps filter, while Figures 17 and

18 show the magnitude and phase response of the

filter at 120 Sps. The Sinc3 is active for all output

word rates except for the 3200 Sps and 3840 Sps

(MCLK = 4.9152 MHz) rate. The Z-transforms of

the two filters are shown in Figure 19. For the Sinc3

filter, “D” is the programmable decimation ratio,

which is equal to 3840/OWR when FRS = 0 and

3200/OWR when FRS = 1.

The converter’s digital filters scale with MCLK.

For example, with an output word rate of 120 Sps,

the filter’s corner frequency is at 31 Hz. If MCLK

is increased to 5.0 MHz, the OWR increases by

1.0175 percent and the filter’s corner frequency

moves to 31.54 Hz. Note that the converter is not

specified to run at MCLK clock frequencies greater

than 5 MHz.

Figure 16. Digital Filter Response (Word Rate = 60 Sps)

-120

-80

-40

120

180

240

300

Frequency (Hz)

-120

-80

-40

040

120

Frequency (Hz)

Flatness

Frequency

-0.01

-0.05

-0.11

-0.19

-0.30

-0.43

-0.59

-0.77

-1.09

-3.13

Figure 17. 120 Sps Filter Magnitude Plot to 120 Hz

-180

-90

180

120

Frequency (Hz)

Figure 18. 120 Sps Filter Phase Plot to 120 Hz

Note:

See the text regarding the Sinc3 filter’s

decimation ratio “D”.

Sinc5

z 80

–

()5

z 16

–

()5

--------------------------

z 16

–

()3

z 4

–

()3

--------------------------

z 4

–

()2

z 2

–

()2

-----------------------

z 2

–

()3

z 1

–

()3

-----------------------

Sinc3

z D

–

()3

z 1

–

()3

-------------------------

Figure 19. Z-Transforms of Digital Filters

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